The term “ultra-high frequency cable” is herein defined as a cable for which the dimensions of the conductors and the dimensions and characteristics of the dielectric between these conductors are adequate so that this cable has a characteristic impedance of constant value in a wide frequency range extending from a few MHz to several GHz. Its structure may be coaxial, shielded bifilar or unshielded bifilar. Such a cable is used for producing the transmission line of the measuring device according to the present invention.
Here, hydrometry refers to the measurement of the humidity content of a solid substance. If the simplest method for measuring this content, consists of picking up a sample of the material, of drying it and measuring its loss of mass, it is not always feasible as it is not always possible or desirable to proceed with such samplings.
To avoid this drawback, a method has been developed for several years, consisting of sending electromagnetic waves into the test material, based on the large dependence of permittivity on humidity content of the material for high frequencies, as permittivity of water is much larger than that of bodies such as clay which it may impregnate. The scientific foundations of the method have been described in several publications, on which we shall not elaborate in detail.
Among these methods, those based on the measurement of permittivity in the high frequency domain, approaching from lower frequency values, the relaxation frequency of water, i.e., 30 GHz at room temperature, may be considered as being close to the invention. They consist of sending a ultra-high frequency signal into a coaxial line, the dielectric of which (for example air) is replaced at the transducer by a material sample (for example clay) in hydric equilibrium with material, the water content of which is intended to be measured. The results are generally provided by comparison with tables of theoretical and/or experimental results.
However a problem occurs in that it is not easy to devise in concrete terms a layout which allows the power conveyed by the incident signal to be distributed among several sensors so that each of the latter sends back a sufficient signal to be analyzed, without picking up an excessive portion of the total signal, and to our knowledge, none of the prior art achievements manages to do this. Indeed, electromagnetic properties of the material to be measured lead to reflection of the quasi-totality of the incident wave, which precludes any mounting of several transducer cells in series. Further, even if one manages to distribute the excitation energy among different transducers, it is very difficult to limit interferences between these potential transducers. This limitation is very disadvantageous, because many applications require that several simultaneous measurements be conducted in different places of a site without having to multiply the equipment used.
In the present state of the art, digitization of signals which have traveled through the ultra-high frequency cable, cannot be directly performed for frequencies ranging up to several GHz. A frequency changing step should then be performed beforehand by techniques known to one skilled in the art (multiplication of one frequency F1 by a frequency F2 and then selection of the frequency band F1-F2).
The electronic means capable of generating sine wave trains at frequencies assuming several values in an arithmetic progression between a few MHz and a few GHz, should be as stable as possible. Preferably they consist of a quartz-stabilized frequency synthesizer. They may optionally consist of a wobbulator which we shall reconsider later.
The measurement signal is applied to vector voltmeters capable of performing the change in frequency, filtering, digitizing, digital filtering, and determining the real and imaginary components of permittivity. A digital processing operation known to one skilled in the art may be added as a complement, notably for correlation with tables of pre-recorded measurements.
A simplified means for achieving excitation and read-out of the signals consists of using a network analyzer, as this will be seen in our detailed description of the operation. Such an apparatus, well known to one skilled in the art, further includes a vector voltmeter VR forming a channel for measuring a reference voltage at the output of the electronic means generating the excitation signal. With such a measurement, it is possible to standardize the signals, i.e., get rid of constant parameters which notably depend on the ultra-high frequency cable and interconnection devices. Finally, a network analyzer has digital computation possibilities.
The separator device capable of sampling from the incident wave only a portion with sufficient energy, is normally designed so as to pick up just sufficient energy so that the measuring cell sends back an echo which may be measured by the electronic read-out means, i.e., a few μW in the present state of the art of measuring apparatuses, on which comments will be made subsequently. More generally, separator devices should be designed so that the proportion of energy which they direct towards the measuring cell is at least equal to the minimum amount of energy which this cell requires.
In reality, each measuring cell does not pick up a constant amount of energy but a constant proportion. And the functional constraint to be observed is to make sure that the cell the most away from the source receives at least the minimum amount of energy ensuring measurement performances. As the ultra-high frequency wave travels through the different measuring cells, its energy decreases and the proportion of this energy picked up by each separator must be changed if an optimized hydrometric measurement system is desired which only picks up the required minimum from this wave.
Now, functionally, if the measuring device comprises many cells, it is obvious that the first cell will only pick up a very small percentage of the incident energy, while the last one may pick up the major portion of it. As the dimensional characteristics of a cell determine the energy percentage which it will pick up, an optimized hydrometric measurement line should include cells which are all slightly different.
Nevertheless, the making of the distributed hydrometric sensor may be simplified by choosing in a suboptimal way to make separator devices which pick up from the ultra-high frequency wave, an amount of energy larger than that which they are normally designed for. A restricted number of dimensional alternatives of the separator devices used or even separator devices which are all identical at each measuring station, may thereby be obtained, which lowers the cost of the whole of the distributed hydrometric sensor.
The making of this separator device may resort to all the known means in the field of ultra-high frequencies, notably to power separators with two very dissymmetrical outputs. In this case, it is sufficient to connect to the lower power output, a simple measuring cell operating as a dead end.
In the other cases, which correspond to the preferential embodiment, this separator which performs the dissymmetric sampling of energy of the ultra-high frequency wave is made by simply juxtaposing dielectric media with different characteristics, and notably of the same nature but with different sections, this for a constant characteristic impedance.
Let us explicit this in the case when the cable is coaxial. Let us call di and de the inner and outer diameters of the dielectric of the ultra-high frequency cable. Diameter di is also the diameter of the core of the cable, and diameter de is also the inner diameter of the peripheral shielding conductor. Let us call d′i and d′e the corresponding diameters of the shrinked cable, and d″i and d″e the corresponding diameters for the measuring cell placed around the shrinked cable. The necessary operating conditions may then be expressed simply by:d′i<di di<d′e<de d″i<de d″e≧de 
Further, the proportion of energy entering the measuring cell will depend on the proportion of dielectric surface of the cell (or if there is a dielectric washer which precedes it, on this dielectric washer) facing the crown-shaped section of the dielectric of the ultra-high frequency power cable, i.e., as a function of the ratio:(π/4)(de2−di2)(π/4)(de2−d″i2)
Analogously, the proportion of energy entering the shrinked portion of the ultra-high frequency cable is a function of the ratio:(π/4)(de2−di2)(π/4)(d′e2−di2)
Moreover, in order to retain the same characteristic impedance, preferentially set to 50 Ohms, the ratio between diameters di and de of the ultra-high frequency cable is the same as the ratio between diameters d′i and d′e of the shrinked cable from the moment that the dielectrics have the same permittivity index.
In the case when the ultra-high frequency cable is bifilar and shielded, transposition is immediate, provided that the outer diameter of the insulator d′e of the shrinked cable is wider than the distance separating the most remote points of both conductors in a transverse cross-section of the main ultra-high frequency cable, this distance may then play the same role as di, although the calculations of the facing sections then have to be corrected accordingly.
In the case when the ultra-high frequency cable is bifilar and unshielded, transposition is immediate, relatively to the previous case. On the other hand, the metal surfaces delimiting the measuring cell remain perfectly connected to each other all around the axis of the cable, but are not electrically connected to anything else.